This invention relates to ferroelectric polymers and, more particularly, to relaxor ferroelectric polymers that have been processed to exhibit high electrostrictive strain levels.
Ferroelectric polymers have attracted interest for many years as they reveal a new aspect of polymers for use as functional materials. Such polymers have been recognized for their potential uses in a variety of large area transducer, actuator and sensor applications because of their flexibility, mechanical strength, light weight, easy processibility into large area films and ability to be arranged into desirable configurations. By contrast, existing functional materials, such as ceramics, are brittle, heavy and difficult to produce into large area configurations.
In spite of their advantages over the ceramics, current polymers suffer low field sensitivities, such as dielectric constant, piezoelectric coefficient, electromechanical coupling factor and field induced strain. These constraints severely limit the application of ferroelectric polymers as transducers, sensors and actuators.
There is a demand for improved materials for use in actuators and transducers due to the limitations of currently available materials. For example, current actuator materials, such as electrostatic, electromagnetic and piezoelectric materials, exhibit limitations in one or more of the following performance parameters: strain, elastic energy density, speed of response and efficiency. For instance, piezoceramic and magnetostrictive materials, while possessing low hysteresis and high response speeds, suffer from low strain levels (xcx9c0.1%). Shape memory alloys generate high strain and high force but are often associated with large hysteresis and very slow response speeds. On the other hand, there are several polymers such as polyurethane, polybutadine etc. which can generate high electric field induced strain i.e. up to 6-11%. But, due to their low elastic modulus, their elastic energy density is very low. Further, the strain generated in these materials is mainly due to the electrostatic effect, which is a low frequency process. Use of these materials at high frequencies reduces their response drastically. In addition, due to their low dielectric constant, the electric energy density and electromechanical coupling coefficient of these polymers is very low which is an undesirable characteristic for many transducer and actuator applications.
Substantial efforts have been devoted to improvement of phase switching materials where an antiferroelectric and ferroelectric phase change can be field induced to cause a high strain in the material. While strains higher than 0.7% have been achieved in such materials, due to the brittleness of ceramics, severe fatigue has been found to occur at high strain levels. Recently, in a single crystal ferroelectric relaxor, i.e., PZN-PT, an electric field strain of about 1.7%, with very little hysteresis, has been reported, which is exceptionally high for an inorganic materials (see: Park and Shrout, J Appl. Phys., 82, 1804 (1997)). In such ceramic materials, mechanical fatigue occurs at high strain levels, a major obstacle limiting their use for many applications.
For many applications, such as microrobots, artificial muscles, vibration controllers, etc., higher strain levels and higher energy densities are required. Thus, there is a need for a general purpose electroactive material with improved performance for use with transducer and actuators.
There is a further requirement for improved ultrasonic transducers and sensors for use in medical imaging applications and low frequency acoustic transducers. Current piezoceramic transducer materials, such as PZTs, have a large acoustic impedance (Z greater than 35 Mrayls) mismatch with the air and human tissue (Z less than 2 Mrayls). On the other hand, piezoelectric polymers such as P(VDF-TrFE), PVDF not only have an acoustic impedance well matched (Z less than 4 Mrayls) to human tissue but also offer a broad nonresonant frequency bandwidth. But, because of their low piezoelectric activity and low coupling coefficient, the sensitivity of such ultrasonic polymer transducers is very low.
The capacitor industry also requires a capacitor which has a much higher electric energy density than is currently available. Current dielectric materials, such as polymers, have a low dielectric constant (xcx9c2-10) and limited energy density. In addition, with current ceramics, the maximum field which can be applied is limited.
Accordingly, it is an object of the invention to provide a polymeric material which can generate a high electric field-induced strain with little hysteresis.
It is another object of the invention to provide a polymeric material which exhibits a high elastic energy density.
It is a further object of the invention to provide a polymer in which the direction of induced strain can be tuned by means of alteration of the ratio of transverse strain (S1) to longitudinal strain (S3).
It is yet another object of the invention to provide a polymeric material that exhibits a room temperature dielectric constant that is higher than other currently available polymers.
It is a further object of the invention to provide a polymer which exhibits relaxor ferroelectric behavior and hence has a slim polarization hysteresis loop which, coupled with high electric field breakdown strength, can provide a capacitor with high electric energy density.
The invention is embodied in an electrical device which includes at least a layer of a ferroelectric polyvinylidine fluoride polymer that has been processed to exhibit an electrostrictive strain of 4% or more when an electric field strength of 50 megavolts per meter or greater is applied thereacross. The processing of the polymer preferably involves subjecting it to either electron beam radiation or gamma radiation. The polyvinylidine fluoride polymer is selected from the group of: polyvinylidine fluoride, polyvinylidine fluoride-trifluoroethylene P(VDF-TrFE), polyvinylidine tetrafluoroethylene P(VDF-TFE), polyvinylidine trifluoroethylene hexafluoropropylene P(VDF-TFE-HFE) and polyvinylidine hexafluoropropylene P(VDF-HFE).